siliconchip.com.au Australia’s electronics magazine July 2019 93
... Screen25: the centre
frequency adjustment
screen. Select a frequency
from 15Hz to 15kHz
using the rotary encoder,
then press S2 to...Screen26: adjust the gain
or cut for this equaliser
band, from -10dB to
+10dB. Pressing S2
again...Screen27: lets you set the
Q of the filter, to a value
between 0.1 and 10,
which affects how wide
a range of frequencies it
affects.Screen28: back at the
main menu, this time
we’ve selected the Save
option. Pressing S2 brings
us to...Screen29: a screen where
you can choose one of
three settings banks to
save to. Use the rotary
encoder to select one, or
press S1 to abort. Press
S2 or the knob...Screen30: to save the
settings to EEPROM. This
screen is displayed for
a short time, then the
display returns to the
default screen, ie, volume
control mode (Screen03).Screen31: the final option
in the main menu is to
load the settings you
have saved. Bank 0 is
loaded by default at start-
up. To load a different
configuration, select this
option and press S2...Screen32: then select
a bank to load using
the rotary encoder, and
either press S2 to load
it, or S1 to abort and
go back to the volume
control screen.from the LCD screen mounting location that the two boards
will not foul each other.
We attached our control board to the rear of the front
panel using 9mm M3 tapped Nylon spacers, with black
machine screws holding it on at the front and nickel-plated
machine screws at the rear. Ensure that the holes are large
enough to prevent the switches from binding.
You can then attach the rotary encoder knob and connect
the LCD panel and control board back to the CPU board and
power supply board respectively, as per your earlier tests.
Performance
Fig.20 shows the output of a spectrum analyser con-
nected to one pair of outputs on the DSP Active Crossover.
A pure 1kHz sinewave is being fed into the inputs. This
shows up in the spectral analysis as a large spike just to
the left of centre.
The readout below shows that this fundamental signal
measures -9.72dBFS for the left channel and 1.62dBFS for
the right channel. “dBFS” stands for ‘decibels full scale’.
In this case, the full-scale output is around 2.2V RMS, so
those signals are at around 0.72V RMS and 1.8V RMS, re-
spectively.
The smaller spikes you can see to the right of the funda-
mentals, at 2kHz, 3kHz etc are the harmonics, ie, the dis-
tortion products resulting from the signal passing through
the unit. The most significant are at 3kHz and 5kHz, ie, the
third and fifth harmonics.
The software measures the relative levels of each harmon-
ic and the fundamental (first harmonic) and feeds them into
a formula to calculate the total harmonic distortion (THD)
ratio for each channel, which it’s showing as 0.0004% for
the left channel (remember, that’s the one with the reduced
signal level!) and 0.0001% for the right channel.
Note that if you incorporated the noise measurement
(seen in the wiggly bases of the plots), these figures wouldn’t
be quite as good, but they’re vanishingly low either way,
and you certainly won’t complain about the sound quality
coming out of this device.
Using it
The DSP Active Crossover is set up and controlled us-
ing a menu system. Menu entries are shown on the graph-
ical LCD while the rotary encoder and two pushbuttons
are used to scroll through entries, select them and go back
to the start. The various menu screens are shown in the
panels overleaf and on these pages, along with a descrip-
tion of each one.
After showing two splash-screens in quick succession,
the unit defaults to the volume control screen. This allows
you to use it as a preamp, varying the volume with the ro-
tary encoder knob, from -104db up to +12dB (the default
is 0dB). Pressing either button (or the knob, if your rotary
encoder has an integral button) takes you to the main menu,
which has four options.
The rotary encoder selects between those options, while
button S2 or the integral rotary encoder pushbutton selects
the current option. This button is used as an “Enter” key
while button S1, at right, acts as “Escape”, to go back to the
main screen without making any further changes.
Once you’ve selected one of the options, you use S2 to
cycle through the available sub-options and the rotary en-
coder to make changes to those options. SC